Membranes and membrane modules for separation of hydrogen from other gases are known. See Zolandz et al. at pages 95-98 in Membrane Handbook (1992). In particular, useful membranes for hydrogen separations are of four types: polymeric, porous ceramic, self-supporting nonporous metal, and nonporous metal supported on a porous rigid matrix such as metal or ceramic.
Polymeric membranes are commonly used in the form of extended flat sheets or small diameter hollow fibers. Flat sheet polymeric membranes are most often incorporated into spiral-wound modules. In this case, the membrane forms an envelope around a flexible polymeric or cloth net (the permeate spacer). The edges of the membrane are glued together to form a gas-tight seal that separates the feed gas, which flows over the outer surface of the membrane envelope, from the permeate gas, which is collected in the cavity created by the permeate spacer. The permeate spacer forms a continuous channel connecting to a permeate collection tube that allows the permeate hydrogen to flow through the permeate spacer and into the permeate collection tube.
Hollow fiber membranes are incorporated into hollow-fiber modules which are very similar in design to shell-and-tube heat exchangers. Polymeric adhesives and sealants such as epoxy resins are used to seal the tubular or hollow fiber membranes into the module shell to form a gas-tight barrier. This allows the gas to be fed to either the interior or exterior of the fibers, thereby preventing gas from flowing into the permeate stream except by permeation through the fiber wall. In cases where the feed gas is directed to the interior of the fibers the hydrogen permeate is collected on the "shell" side or outside of the tubes or fibers.
Polymeric membranes and membrane modules for hydrogen separations suffer from a lack of high selectivity toward hydrogen over other gases resulting in a relatively impure product gas, a lack of stability at operating temperatures above 250.degree. C., and chemical incompatibility with many chemicals such as hydrocarbons that are present in the impure hydrogen feed stream. To overcome these limitations, highly selective and more robust materials must be used for the hydrogen separation membrane and for sealing the membrane into the membrane module.
Inorganic materials, notably nonporous and porous ceramics and nonporous or dense metals, are known to make robust hydrogen-selective diffusion membranes. Such inorganic membranes are suitable for use at temperatures above 250.degree. C. and are not damaged by many chemicals, including hydrocarbons.
Nonporous inorganic oxides are known to be permeable to hydrogen in its ionic form. For example, U.S. Pat. No. 5,094,927 discloses materials that are permeable to hydrogen ions (referred to as "solid-state proton conductors") based on silicon oxide, oxides of Groups IVB, VB, VIB and VIII of the Periodic Table, and fluorides of Groups IIA and IIIB of the Periodic Table. Additionally, diffusion coefficients for hydrogen ions through the oxides of molybdenum and tungsten have been reported by Sermon et al. in 72 JCS Faraday Trans. I 730 (1976).
Such solid-state proton conductors have been used by placing them between the cathode and anode in fuel cells, resulting in a net transport of hydrogen between the cathode and anode. However, these solid-state proton conductors are generally brittle, exhibit relatively low permeability to hydrogen, and have not generally been reported for use as a hydrogen separation membrane. The one exception is a nonporous silicon oxide membrane that is reported to allow hydrogen permeation through the silicon oxide by an activated surface-transport mechanism along grain boundaries. See Gavalas et al.,44 Chem. Eng. Sci. 1829 (1989). Although this dense silicon oxide membrane exhibits very high selectivities for hydrogen relative to nitrogen, it is also brittle and susceptible to reaction with steam at elevated temperatures, further limiting its utility.
Exemplary materials that have been investigated for use as porous inorganic molecular hydrogen-permeable membranes include aluminum oxide, silicon oxide, titanium oxide, magnesium oxide, chromium oxide, tin oxide, and various zeolites. See, for example, Hsieh, 33 Catal. Rev. Sci. Eng. 1 (1991). While such membranes exhibit very high hydrogen permeability, they also suffer from very low hydrogen selectivity due to their relatively large mean pore diameter and, as with the nonporous hydrogen-permeable ceramics discussed above, porous ceramics are also very brittle by nature and so are susceptible to failure due to cracking.
Porous ceramics, typically alpha- or gamma-aluminum oxide in the form of tubes, separate hydrogen from other gases based on differential gas phase diffusion rates through the pores of the ceramic. Such ceramic membranes are typically incorporated into a shell-and-tube module. A seal between the ceramic tube and the module shell, to prevent the feed gas from flowing directly into the permeate stream, is made by one of two methods: (1) polymeric o-rings are used to make the seal outside of any heated zone of the membrane module; or (2) graphite string or cord is used with metal compression fittings to make the seals within the heated zone of the membrane module. The use of polymeric sealing materials requires that the ends of the membrane module be kept cool, which is difficult when large volumes of gas are flowing through the module. Because these porous ceramic membranes have relatively low selectivity for hydrogen over other gases, the integrity of the seals is often difficult, if not impossible, to assess.
To overcome the inherently low selectivity of porous ceramic membranes, palladium- or palladium-alloy-coated ceramic membranes have been disclosed. See Hsieh, "Inorganic Membrane Reactors," 33 Catal. Rev Sci. Eng. 1 (1991). Since nonporous or dense layers of hydrogen-permeable metals such as platinum, palladium, nickel and certain alloys thereof are permeable only to hydrogen, the selectivity for hydrogen over other gases is very high, which is a desirable characteristic of membrane-based separations. Such metal-coated ceramic membranes are typically incorporated into shell-and-tube modules using graphite gaskets within a compression fitting to seal the membrane tube to the module, thereby to prevent gas flow from the feed stream directly to the permeate stream. However, the large differences between the coefficient of thermal expansion of the ceramic tube and of the metal compression fitting, combined with the brittleness of the ceramic tube, results in a high frequency of leaks between the feed stream and the permeate stream at the gasket. See J. P. Collins, "Preparation and Characterization of a Composite Palladium-Ceramic Membrane," 32 Ind. Eng Chem. Res 3006 (1993). Another drawback of ceramic-supported thin metal foil membranes is that the metal foil is subject to macroscopic ruptures should the ceramic crack due to uneven loading or to thermal or mechanical shock.
Nonporous metal membranes that are selectively permeable to hydrogen are also known. See, for example, U.S. Pat. Nos. 4,388,479 and 3,393,098, both of which disclose Group VIIB and VIII alloy membranes such as palladium alloy catalytic membranes. Such metal membranes are superior to polymeric membranes and to inorganic (non-metal) membranes in that they have essentially complete selectivity for hydrogen over other gases, can be operated at temperatures up to about 1000.degree. C., and are chemically resistant to gases in the feed stream. However, the prohibitively high cost of palladium has led to efforts to fabricate composite hydrogen-permeable metal membranes by coating certain less expensive transition metal alloy base metals with palladium or palladium alloys. See, for example, U.S. Pat. Nos. 4,468,235 and 3,350,846. The palladium or palladium-alloy coating on such base metals employs only a relatively small amount of palladium, imparting chemical resistance to the base metal and in some cases increasing the rate of adsorption of hydrogen onto the metal membrane surface.
U.S. Pat. No. 2,958,391 describes a metal membrane module consisting of a palladium or palladium alloy supported directly on a porous base metal comprising a sintered-metal matrix. The sintered-metal matrix may be in the shape of a flat plate or an elongated cylinder. Hydrogen permeates from the external surfaces of the palladium or palladium alloy membrane into the porous sintered-metal matrix, is conducted through its pore structure, and is collected.
In addition to porous ceramic and sintered-metal supports for hydrogen-permeable metal membranes, U.S. Pat. Nos. 3,477,288 and 4,699,637 disclose the use of a metal mesh or gauze to support the thin metal membrane. Means to fabricate membrane modules are not taught in these patents. However, Canadian Patent No. 1,238,866 describes the use of a silver-based solder to seal to the module the edges of a flat-sheet palladium alloy membrane supported on a metal mesh or gauze, porous sintered metal, or perforated metal.
However, such coated or supported metal membranes have an inherent shortcoming in that, under the elevated temperature conditions of use, the coating metal tends to diffuse into the base metal or porous metal support, thereby destroying both the hydrogen permeability and the chemical resistance available from such composite metal membranes. U.S. Pat. No. 4,496,373 discloses a nonporous hydrogen-permeable composite metal membrane that addresses this intermetallic diffusion problem for a base metal alloy of a specific composition coated with a palladium alloy of specific composition. However, the composition of the palladium alloy coating and the base metal alloy are narrowly defined so as to favor partitioning of the palladium into the coating alloy as opposed to the base metal alloy. Consequently, this approach is not general in nature, requires strict control over alloy composition, and allows for little variation in selection of metals for membrane fabrication.
A general approach to preventing intermetallic diffusion in composite metal membranes, disclosed in commonly owned U.S. Pat. No. 5,259,870 and application Ser. No. 08/148,999, is to utilize a chemically and thermally stable intermediate layer between a coating metal layer and a dense hydrogen-permeable base metal. The coating metal layer comprises a dense (i.e., non-porous), hydrogen-permeable metal including palladium and palladium alloys. The base metal layer is also a dense, hydrogen-permeable metal and is selected from the metals found in Groups 3 through 5 of the Periodic Table and their hydrogen-permeable alloys. The intermediate layer (also called the intermetallic diffusion barrier) includes chemically and thermally stable oxides (e.g., aluminum oxide and silicon oxide) and serves to prevent direct contact between the coating metal layer and the base metal layer.
Japanese Laid-Open Application Nos. 346,824/92 and 76,738/93 both disclose a hydrogen gas separation membrane comprising a thin membrane of palladium, a porous metal support and a ceramic or metal oxide barrier layer between the palladium and the support. However, the barrier layer is inherently rigid and brittle.
PCT application No. 90/09231 discloses a hydrogen separation membrane comprising an inorganic support having interstices, the interstices of the support being bridged by a composite layer of partially sintered non-metallic particles and a hydrogen-permeable metal such as palladium, the bridging taking place in such a fashion as to render the composite layer coplanar with the support.
In all of these approaches to using an oxide layer to limit or prevent intermetallic diffusion in a composite metal membrane, the oxide layer is inherently brittle. Thus, membranes made according to these teachings are subject to failure due to formation of pinholes, cracks, and/or tears in the coating metal layer as a result of fracture of the brittle oxide layer directly beneath the coating metal layer.
The use of a chemically reactive intermediate oxide layer in hydrogen-permeable metal membranes is also known. In contrast to the chemically and thermally stable intermediate layers described above, such a reactive oxide layer facilitates, rather than prevents, intermetallic diffusion. For example, Russian Patent No. 1,058,587 discloses a method for manufacturing membrane elements for diffusion-based hydrogen separators by diffusion-welding palladium or palladium-alloy membranes to an undefined metal substrate. Specifically, the '587 patent discloses first saturating a hydrogen-permeable coating metal at elevated temperature, then cooling the so-hydrogen-loaded coating metal, then applying a "reactive gasket" of ultrafinely divided powders of metallic oxides over the area between a base metal and the coating metal where the base and coating metals are to be welded together, then subjecting the composite to high pressure (2000-2500 psi) and high temperature (650.degree.-700.degree. C.) to achieve a "diffusion weld" between the coating metal and the base support metal. The diffusion weld results from the complete reduction of the metal oxides "reactive gasket" intermediate layer to pure metal(s) by hydrogen desorbed from the hydrogen-loaded coating metal. It is unclear whether (1) the palladium or palladium-alloy membrane is attached only to the edges of the metal substrate via the diffusion-bonded weld, or (2) the palladium or palladium-alloy membrane completely covers the surface of the metal substrate and the diffusion-bonded weld. In the first case, the welded portion of the membrane need not be hydrogen-permeable, as hydrogen is required only to permeate the unwelded portion of the palladium or palladium-alloy membrane and the hydrogen-permeable portion of the membrane is not a composite metal membrane at all, but rather is simply a palladium or palladium-alloy membrane. The drawback of such an approach is that the palladium or palladium-alloy membrane must be sufficiently thick to be self-supporting and the membrane is therefore unacceptably expensive. In the second case, the resulting composite membrane would include an intermediate layer which, after fabrication, is a metal or metal alloy, with attendant reduction in the overall hydrogen permeability of the membrane.
Despite the fact that hydrogen-permeable metal membranes were first commercialized nearly three decades ago, practical and affordable metal membrane modules are still lacking. Known module designs suffer from high cost due to complex configurations and permanent assembly methods that make repairs difficult and expensive, reduced membrane permeability due to interdiffusion of metallic constituents from the metal support matrix or from the module itself, and breakdown of the membrane due to damage to the coating metal layer arising from dimensional changes in the membrane under the conditions of use. The present invention overcomes these and other shortcomings of the prior art.